Revisiting the Admission of App

Here is the
definition of app again with certain parts highlighted. If you
are taking the Walking Tour, please read the text carefully and
click on each of the links below, except those marked.
Then come back here.

The admission of APP is trivial, using the
relation O< (which is known to be well-founded on
the domain recognized by O-P) and the measure
(ACL2-COUNT X). We observe that the
type of APP is described by the theorem (OR
(CONSP (APP X Y)) (EQUAL (APP X Y) Y)). We used primitive type
reasoning.